Venting arrangement for storage tanks

ABSTRACT

Petroleum storage tanks or the cargo tanks of a tankship have an upwardly directed minimum velocity escape valve in the vent line or lines. During filling this ensures that no flammable concentration of petroleum vapors forms below the valve, e.g. at the deck level of a tankship. A suitable valve has a tapered sleeve which cooperates with a pointed core. The sleeve lifts to provide a larger annular opening to prevent too high pressure drop at high mass flow rates and falls to reduce the opening to keep a high velocity at lower mass flow rates.

United States Patent Inventors Appl. No. Filed Patented AssigneePriority Will Pratt Kenley, Surrey;

Geoffrey Joseph Grocott, Bearsted, Maidstone, Kent, England Oct. 8, 1968Jan. 26, 1971 The British Petroleum Company Limited London, England Oct.10, 1967 Great Britain VENTING ARRANGEMENT FOR STORAGE TANKS 4 Claims, 3Drawing Figs.

U.S.Cl 114/211, 137/533, 220/44 Int. Cl B63j 2/14 Field of Search 114/21 1,

212; 62/50; 98/66, 53, 76, 77, 79,116; 137/533.17, 533, 508, 511,533.19(lnquired); 220/44 [56] References Cited UNITED STATES PATENTS779,157 l/ 1905 Gronwald 144/212 3,279,198 10/1966 Ayers et al 144/2113,334,667 8/1967 Hedstrom 62/50(X) 2,833,299 5/1958 Marriette 137/508(X)Primary ExaminerMilton Buchler Assistant ExaminerJames E, PittengerAttorney-Morgan, Finnegan, Durham & Pine PATENTEU JAN26 I971 SHEET 1 [IF3 VENTING ARRANGEMENT FOR STORAGE TANKS This invention relates toventing arrangements for storage tan ks which are used to containvolatile liquids which produce flammable vapors. In particular itrelates to venting arrangements for the cargo tanks of tankships.

In general, the tanks of tankers which have discharged their cargos areventilated to remove flammable vapors during an empty passage. Whentankers are loading crude petroleum or products obtained therefrom thereis a fire hazard in that flammable vapors arise because a volatile cargoattempts to vaporize as soon as it enters an empty tank and there issufficient vaporization to represent a fire hazard when the airoriginally present in the tank and contaminated by vapor during loadingis discharged from the vent pipe as loading proceeds. The discharge ofthis vapor constitutes a hazard in that flammable (which includesexplosive) mixtures of petroleum vapors and air may form at deck level.A similar fire hazard occurs when large storage tanks of landinstallations are refilled with volatile, flammable materials. It is anobject of the present invention to reduce this hazard.

According to the invention a storage tank is provided with an upwardlydirected minimum velocity escape valve in every vent line, each minimumvelocity escape valve being so constructed that dangerous quantities offlammable vapor are upwardly directed with sufficient velocity toprevent the formation of a flammable air/vapor mixtures at or below thelevel of the vent.

The invention is particularly suitable for application to tankships anda tankship according to the invention is provided with an upwardlydirected minimum velocity escape valve in every vent line to its cargotanks, each minimum velocity escape valve being so constructed thatdangerous quantities of flammable vapor are upwardly directed withsufficient velocity to prevent the formation of a flammable air/vapormixture at deck level.

Every cargo tank may be provided with its own separate vent line and inthis case the minimum velocity escape valves are conveniently situatedat or near deck level. Alternatively several or all the cargo tanks mayshare the same vent line and in this case the minimum velocity escapevalves (or valve when all the cargo tanks share the same vent line) arepreferably situated as high above the deck as possible, eg at mastheadlevel.

The arrangement described above ensures sufficient velocity of escapefor petroleum vapors without creating undesirable pressure drops in thevent lines.

A suitable form of minimum velocity escape valve comprises a slidingmember having a tapered, e.g. frustoconical portion, which cooperateswith a pointed core to define a a variable aperture, the slidablemember, in the use of the valve, being lifted by the flow of gas wherebysufficient escape velocity is maintained without creating undesirablepressure drop.

The invention will now be described by way of example with reference tothe accompanying drawings in which:

FIG. 1 is a diagrammatic cross section of a tank vessel;

FIG. 2 is a perspective, cutaway view of a minimum velocity escapevalve; and

FIG. 3 is a vertical cross section through a valve similar to the oneshown in FIG. 2.

The tankship illustrated in FIG. 1 comprises cargo tanks whoseindividual vents 11 are all connected to a common header 12 whichconnects to a common vent line in the hollow interior of the mast l3terminating in a minimum velocity escape valve 14 situated at themasthead. Alternatively the common header 12 may be abolished andseparate minimum velocity escape valves fitted in every vent line 11.

During loading, which may take place at 5,000 tons per hour,considerable quantities of petroleum vapors are displaced from the tanksand all of these are discharged via the minimum velocity escape valve 14(or valves for the altemative system) which ensures a gas dischargevelocity of at least 150 fps. in an upward direction and this velocityis sufficient to ensure that dangerous quantities of vapor do notdiffuse back to deck level. If the rate of loading becomes very low, andhence the rate of volume discharge of petroleum vapors becomes low, thelinear velocity of escape may fall well below I50 f.p.s. in spite of theminimum velocity escape valve. However this only happens at such lowrates of loading that the total quantity of vapors discharged is notsufficient to create a hazardous concentration at deck level.

The constant velocity escape valve 14 of FIG. I is shown in greaterdetail in FIG. 2. It comprises a core member 20 whose upper end isprovided with a conical point 21. The core member 20 is surrounded by aslidable member 22 which has a frustoconical portion 23 which cooperateswith the conical point 21. The whole arrangement is surrounded by acylindrical shield 24. 2

During loading the slidable member 22 is lifted by the flow of gas andthe greater' the flow of gas the further the slidable member 22 islifted thereby providing a greater effective opening. This preventsundesirably high pressure drops with high rates of discharge. On theother hand when the rate of discharge falls there could be a danger ofpetroleum vapors diffusing back to deck level but the slidable member 22falls thereby decreasing the effective aperture. This has the effect ofmaintaining the pressure drop and also the velocity of discharge so thatthere fail no danger of petroleum vapors diffusing back to deck level inspite of the decreased rate of discharge. As has been mentioned abovethe valve may fail to maintain high velocities for rates of dischargewhich are too small to constitute a hazard.

In the modification shown in FIG. 3 the slidable member 22 is attachedto the shield 24 by means of extendable cylindrical bellows 25. Thisdoes not alter the basic operation of the valve but it ensures that allthe gas flows through the slidable member 22. To ensure adequatecentering the slidable member 22 carries tapered guide vanes 26. (Sincethese are thin plates parallel to the direction of flow they offersubstantially no resistance.)

For protection the valve has a lid 27 (shown in the open position). Whenthe valve is not in use, e.g. at sea, the lid is closed so that theoperative parts of the valve are not open to sea or weather.

We claim:

1. In a storage tank for volatile liquids which give rise to flammablevapors, said tank being provided with at least one vent line for theescape of vapors while refilling, the improvement of an upwardlydirected minimum velocity escape valve in every vent line, each minimumvelocity escape valve comprising a fixed pointed core member and aslidable member surrounding said core member and having a taperedportion, said tapered portion cooperating with the point of said coremember to define a variable aperture for the escape of vapor, wherebydangerous quantities of flammable vapor are upwardly directed withsuflicient velocity to prevent the formation of a flammable air/vapormixture at the level of the vent.

2. In a tankship for carrying volatile liquids which give rise toflammable vapors, said ship having a plurality of cargo tanks and aventing system which provides a plurality of vent outlets for the escapeof vapors from the cargo tanks while loading, the improvement of aplurality of minimum velocity escape valves one of which is positionedin each vent outlet, each minimum velocity escape valve comprising afixed pointed core member and a slidable member surrounding said coremember and having a tapered portion, said tapered portion cooperatingwith the point of said core member to define a variable aperture for theescape of vapor, whereby dangerous quantities of flammable vapor areupwardly directed with sulficient velocity to prevent the formation of aflammable air/vapor mixture at deck level.

3. In a tankship for carrying volatile liquids which give rise toflammable vapors, said ship having a plurality of cargo tanks and aventing system which connects all of said tanks to a single vent outletfor the escape of vapors from the cargo tanks while loading, theimprovement of a minimum velocity escape valve situated in said outletand comprising a fixed with sufficient velocity to prevent the formationof a flammable air/vapor mixture at deck level.

4. A tankship according to claim 3, in which the minimum velocity escapevalve is situated at masthead level.

1. In a storage tank for volatile liquids which give rise to flammablevapors, said tank being provided with at least one vent line for theescape of vapors while refilling, the improvement of an upwardlydirected minimum velocity escape valve in every vent line, each minimumvelocity escape valve comprising a fixed pointed core member and aslidable mEmber surrounding said core member and having a taperedportion, said tapered portion cooperating with the point of said coremember to define a variable aperture for the escape of vapor, wherebydangerous quantities of flammable vapor are upwardly directed withsufficient velocity to prevent the formation of a flammable air/vapormixture at the level of the vent.
 2. In a tankship for carrying volatileliquids which give rise to flammable vapors, said ship having aplurality of cargo tanks and a venting system which provides a pluralityof vent outlets for the escape of vapors from the cargo tanks whileloading, the improvement of a plurality of minimum velocity escapevalves one of which is positioned in each vent outlet, each minimumvelocity escape valve comprising a fixed pointed core member and aslidable member surrounding said core member and having a taperedportion, said tapered portion cooperating with the point of said coremember to define a variable aperture for the escape of vapor, wherebydangerous quantities of flammable vapor are upwardly directed withsufficient velocity to prevent the formation of a flammable air/vapormixture at deck level.
 3. In a tankship for carrying volatile liquidswhich give rise to flammable vapors, said ship having a plurality ofcargo tanks and a venting system which connects all of said tanks to asingle vent outlet for the escape of vapors from the cargo tanks whileloading, the improvement of a minimum velocity escape valve situated insaid outlet and comprising a fixed pointed core member and a slidablemember surrounding said core member and having a tapered portion, saidtapered portion cooperating with the point of said core member to definea variable aperture for the escape of vapor, whereby dangerousquantities of flammable vapor are upwardly directed with sufficientvelocity to prevent the formation of a flammable air/vapor mixture atdeck level.
 4. A tankship according to claim 3, in which the minimumvelocity escape valve is situated at masthead level.